Gas turbines may be configured with a single combustion chamber, but they may also have what is known as sequential combustion. In the case of the latter, fuel is burned in a first combustion chamber and then the combustion air is allowed to expand via a first turbine, a high-pressure turbine. Downstream of the high-pressure turbine, the still hot combustion gases flow through a secondary combustion chamber, in which fuel is additionally supplied and typically burned by spontaneous ignition. Arranged downstream of the secondary combustion chamber is a low-pressure turbine, through which the combustion gases are allowed to expand, possibly followed by a heat recovery system with steam generation.
The transition from combustion chamber to turbine is in this case a critical region, because particularly complex temperature and pressure conditions exist in this region. The combustion chamber, which is formed for example as an annular combustion chamber, typically has an as it were dish-shaped outer limitation, an outer wall, which consists of a heat-resistant material, or is correspondingly coated, and which is normally made up of individual segments. On the opposite, inner side, lying closer to the axis, there is a correspondingly formed inner limitation, an inner wall, of corresponding materials.
The turbine for its part has a number of alternately arranged rows of guide vanes and rotating blades. The first row of vanes, arranged directly downstream of the combustion chamber, is typically a row of guide vanes with considerable twisting of the vanes with respect to the direction of the main axis. The guide vanes are in this case typically formed as segment modules, in which each guide vane has on the inner side an inner platform and on the outer side an outer platform, and these platforms subsequently also limit with their inner surface the flow channel of the combustion air radially inward and radially outward. On the radially inner side of the annular flow channel there is correspondingly a gap between the inner combustion chamber heat shield (wall segment of the combustion chamber) and the inner platform of the first row of guide vanes, and on the radially outer side there is a gap between the outer combustion chamber heat shield (wall segment of the combustion chamber) and the outer platform of the first row of guide vanes.
It is known from US 2009/0293488, which is incorporated by reference, that it is possible to close this transitional region essentially by a gap of a very small size and additionally provide specific structures which ensure optimum cooling of the wall regions in this area. However, a problem of this approach is that the gap of a correspondingly small size also does not necessarily ensure the required play between the combustion chamber module and the turbine.
On account of the different mechanical and thermal loads on the components that are the combustion chamber and the turbine, however, this gap must have a certain width and cannot be simply closed or completely bridged.
In fact, the thermal expansion of the different components adjacent one another in this region (turbine, combustion chamber) is extremely different and, as a result of the size of the components, is also great in absolute terms. At the interfaces there are correspondingly large gaps, which must have sufficient gap widths over the entire transient stage (for example hot restarting). As a consequence, the gap width at the base-load point, for example, is and must be greater than is necessary for the operating state. Correspondingly, the problem also cannot be readily solved by reducing the gap width.
Furthermore, there are differences in the components and how they can be influenced by the flow processes. To be specific, in the combustion chamber there are only small differences in pressure, while in the region of the turbine there are great differences in pressure due to the vanes, which produce the pressure field. The pressure field acts on the gaps. The parts carrying hot gas outside the flow path must be protected from hot gas. Pressure peaks of the pressure field determine the pressure that must be available in the adjacent cavities. Leakage and higher RTDF (radial temperature distribution function)/emissions are the consequence. The purging of the cavities is actually determined by the pressure peaks occurring, and not by the average pressure.
A problem with this gap, which forms a cavity directed radially away from the hot gas channel and extending into further structural components of the housing, is therefore also the fact that it is additionally exposed to complex flow conditions, in particular in the region of each guide vane. To be specific, a so-called bow wave forms at the leading edge of the guide vanes and has the effect that hot combustion air in the wall region is forced under pressure into this cavity and penetrates into it to a corresponding depth. This can cause problems in connection with overheating, but also with oxidation of the corresponding surfaces.
Moreover, the bending moment loads on the vanes occur at the transitions between the platform and the profile. These bending moments in combination with the thermal stresses restrict the size of the platforms, and therefore the distance within which the gaps can be placed away from the profile stagnation points. In other words, the extents to which the platform can overhang are restricted.